1. Field of the Invention
The present invention relates to a straddle-type vehicle having a V-belt continuously variable transmission (CVT) disposed below a seat of the vehicle and for transmitting a driving force of an engine to a driving wheel, and relates to a power unit including an engine and a V-belt CVT attached to a side of a crankcase of the engine.
2. Description of the Related Art
Recently, there is an increasing demand for straddle-type vehicles called an “ATV” (all-terrain vehicle).
Also, there have been proposed various types of ATVs in which four wheels are provided, each wheel having a wide and low-pressure balloon tire or the like on the left and right sides of the front and rear sides of a body frame, the upper portion of the body frame is provided with steering handlebars, a fuel tank, and a straddle-type seat, arranged sequentially from the front wheel side to the rear wheel side, and further including a V-belt CVT for transmitting the driving force of an engine to driving wheels disposed below the seat (see JP-A-2004-156657, for example).
The above-described V-belt CVT includes a primary sheave disposed on a primary sheave shaft, to which a driving force of the engine is input, and having a movable sheave half and a fixed sheave half to form a V-groove for receiving a belt; a secondary sheave disposed on a secondary sheave shaft, from which a driving force for the driving wheels is output, and having a movable sheave half and a fixed sheave half to form a V-groove for receiving a belt; an endless V-belt received in the respective V-grooves of the primary sheave and the secondary sheave to transmit a rotational driving force between both the sheaves; and a sheave drive mechanism for displacing the movable sheave half of the primary sheave in the axial direction to control the speed change ratio through resulting variations in respective groove widths of the primary sheave and the secondary sheave.
A previous type of sheave drive mechanism, in general was the so-called centrifugal type, which utilized centrifugal force produced according to the engine speed to displace a movable sheave half in the axial direction.
However, when the road on which the vehicle is running turns from a flat road to a climbing road, for example, the sheave drive mechanism of the centrifugal type does not perform a groove width control operation (specifically, a speed change operation) responsively according to changes in the road condition, irrespective of the intention of the rider, until the engine speed has actually decreased because of the increased load from the road. That is, the sheave drive mechanism has a problem of delayed response.
In order to solve the problem of delayed response, another type of V-belt CVT for use in scooter-type motorcycles has been proposed, which includes an electric motor, a control device for controlling the electric motor, and a sheave drive mechanism for transmitting a driving force of the electric motor to the movable sheave of the primary sheave to control the respective groove widths of the primary sheave and the secondary sheave (see JP-B-3043061, for example).
In general, the scooter-type motorcycle is provided with an integral power unit including an engine and a V-belt CVT attached to a side of a crankcase of the engine. In such a scooter-type motorcycle, the power unit may be swingably suspended from a body frame to function as a swing arm for swingably supporting a rear wheel.
With the increasing demand for ATVs these days, there is a need for V-belt CVTs for use in ATVs to electrically control the respective groove widths of the primary sheave and the secondary sheave in order to improve the response in the speed change process.
However, since ATVs are significantly different in structure from scooter-type motorcycles, the installation method of an electric motor as used in V-belt CVTs of scooter-type motorcycles cannot be used in ATVs.
For example, in the V-belt CVT of the power unit also used as a swing arm in scooter-type motorcycles as disclosed in JP-B-3043061, there is a large distance between the shafts of the primary sheave and the secondary sheave and the space between the primary sheave and the secondary sheave can be used as an installation space for an electric motor for controlling the respective groove widths of the primary sheave and the secondary sheave. In contrast, in ATVs, it is difficult to dispose an electric motor between the primary sheave and the secondary sheave as disclosed in JP-B-3043061, because the limitation on the size of a belt chamber due to the arrangement in which the V-belt CVT is disposed between the front wheels and the rear wheels necessitates a compact arrangement in which the primary sheave and the secondary sheave are located close to each other, generally side by side.
In addition, in ATVs, the belt chamber is large in the vehicle width direction because of a footrest for the rider provided on the outer side of the crankcase and the V-belt CVT, and the electric motor, when carelessly arranged on the outer side of the vehicle body, would prevent the rider from putting his/her foot in place, thereby interfering with the usability of the ATV.
Further, in the case where the electric motor is installed near a heat producing portion such as a cylinder block of the engine, the service life of the electric motor and/or the measurement accuracy of the electric motor may be reduced, whereby the original performance cannot be maintained.